1. Technical Field
The present invention relates generally to plain bearings and to methods of their manufacture. More particularly, the invention relates to multilayer engine bearings of the type having a metal backing on which a functional bearing layer is applied which in turn is plated with an overlay of soft metal. Most particularly, the invention relates to multilayer bearings having a functional bearing layer having a surface layer of hard particles embedded therein which is overlaid by a layer of a soft metal, and optionally, with a diffusion barrier layer between the soft metal layer and the functional bearing layer.
2. Related Art
Plain engine journal-type bearings for use in high load engine applications for journaling crank shafts and the like typically include a bearing lining layer having a functional bearing layer of either copper-lead or aluminum alloys applied to a steel backing. A single layer of a lead-tin-copper alloy having a thickness of about 25 μm is typically overplated onto the functional layer. Often, a nickel diffusion barrier or copper bonding layer is interposed between the overplate and functional layer to prevent the tin from diffusing from the overplate into the bearing layer. As a final step, the bearing may optionally be coated with a micro-thin layer of tin or lead-tin flash plating having a thickness of about 1 μm or less. The flash plating is both functional and aesthetic, giving the product a protective outer layer and a bright, pleasing appearance. It also provides a level of corrosion protection to the steel backing. Within a short time during the break-in period of the engine, the flash plating applied to the running surface of the bearing is worn away by the action of the components being born by the bearing.
In service, such multilayer crank shaft bearings are subjected to high dynamic loads that vary in-magnitude and direction due to the inertial loads applied by the piston and connecting rod mechanism and by the combustion gases. The soft overplate layer enables such bearings to conform under high load forces to any misalignments or changes in profile of loading of the member being journaled, so that the loads are distributed across a greater surface area of the bearing. The overplate also allows any foreign particles of dirt or metal that may come between the bearing surface and the member being journaled to become embedded or absorbed into the bearing surface, so as to protect both the bearing and the component being journaled from excessive wear or damage.
It is generally accepted that conformability and embedability of the overplate are dependent upon overplate thickness, with a thicker overplate being preferred. It is also generally known that as the thickness of the overplate increases, so does the susceptibility to bearing fatigue (i.e., the fracturing of the bearing surface when under load). Resistance to fatigue cracking requires that the bearing surface possess sufficient tensile strength to enable it to undergo minor configuration changes without cracking. However, it is also generally known that certain alloys which would otherwise exhibit improved strength and resistance to fatigue, are also more susceptible to sliding wear processes and seizure of the bearing layer to the journaled component, such as the crank shaft, during operation of the engine. Thus, it is has been necessary to balance the properties of conformability and embedability with fatigue resistance of the bearing layer when designing engine bearings, particularly ones that are subjected to high dynamic loading.
For many high load engine applications, it has been found that the 25 μm thick single layer lead-tin-copper alloy overplate mentioned above provides excellent conformability and embedability characteristics and good fatigue resistance of typical bearing layers. However, as the output and efficiency of engines increases, so does the dynamic loading placed on the crank shaft bearings, and thus the increased potential for bearing fatigue. Under extreme loading conditions, the conventional bearings described above employing the single 25 μm thick overplate of lead-tin-copper may be prone to fatigue. Efforts to alleviate fatigue by simply decreasing the thickness of the single layer overplate to less than 25 μm have been generally unsuccessful, as it is at the cost of sacrificing acceptable levels of conformability and embedability. As performance requirements increase, there exists a need in the industry for an improved journal bearing that can perform under such extreme high dynamic loading conditions by providing a sufficient thicknesses of the soft overlay to promote acceptable conformability and embedability while at the same time also enabling the use of stronger more fatigue resistant bearing alloys which also possess the necessary wear properties and resistance to seizure of the bearing layer.